It is well known that plastic material has tendency to creep or shrink when heated and this is especially true of oriented plastic containers such as bottles. With plastic containers this is undesirable because bottles which have shrunk excessively cause trouble in automatic filling systems. Friendship, U.S. Pat. No. 3,886,253 (1975) and U.S. Pat. No. 3,862,288 (1975) both recognize this problem and have suggested placing formed containers in an oven through which is passed heated air in order to stabilize the volume of the bottles. This treatment, while partially satisfactory, suffers from the fact that the entire bottle is treated. Treatment of the less oriented portions of the bottle is undesirable for many reasons. First, there may be loss of definition of design features, and, further, heating of the less oriented portions may result in a loss of clarity of bottles made of clear thermoplastic materials. Finally, a long treatment time is necessary on the order of one half hour. Another approach to the problem is shown in U.K. Specification No. 1,474,044 (1977) wherein formed containers are maintained under pressure in the forming zone. The pressure restrains shrinkage while heat setting the product. The treatment time disclosed in the Specification is 25 seconds.
In view of the fact that bottles as disclosed in the Specification are molded in a cycle time of less than six seconds, the additional time for heat setting drastically reduces the production of the molding apparatus.
A study of temperatures reached in the transportation of polyethylene terephthalate bottles during the summer has been made. In preliminary work, a single pallet was placed in a closed, insulated truck which was parked outside. The pallet contained eight, 64-ounce bottles to a box and ten (10) boxes per layer with six (6) layers on the pallet. A shrinkable plastic wrapping surrounded the pallet sides; the bottom was not wrapped and a single layer of corrugated cardboard was placed on top. This is a typical shipping unit. During a three-day experiment in early summer it was determined that maximum temperature reached was 109.degree. F. for the center of the top layer. The maximum outside temperature was 85.degree. F. and a head space temperature of 122.degree. F. was reached. In another run, a fully loaded truck made a trip of over twenty-four (24) hours during the summer and then parked outside in an unshaded area for three additional days. In this test, maximum bottle temperatures varied between 106.degree. F. and 122.degree. F.
One bottle produced by the assignee of this invention has specifications for a sixty-four ounce bottle as shown in the following table:
______________________________________ Target Minimum Maximum ______________________________________ Summer 1940 cc. 1912 cc. 1948 cc. Winter 1940 cc. 1923 cc. 1948 cc. ______________________________________ Note: Summer defined as calendar months with average temperatures 60.degree. F. and above. Winter defined as calendar months with average temperatures below 60.degree. F.
This means that in summer specifications concerning volume variations from a 1940 cc. volume are +8 down to -28 cc. and in winter +8 cc. down to -17 cc. Our findings have shown that on the average our bottle has a volume of 1930 cc. and at temperatures of 140.degree. F. (relative humidity 10%) for four hours displays a shrinkage of 65 cc.
Work to be disclosed hereafter will show that the untreated bottle exceeds the allowable shrinkage while those treated to the present invention are satisfactory.